1. Technical Field
The technical field relates generally to hybrid motor vehicles and, more particularly, to an operator interface for control over braking response of a hybrid vehicle equipped for regeneration braking and compression braking.
2. Description of the Technical Field
The application of parallel hybrid internal combustion/electric drivetrains to commercial or heavy duty vehicles raises operator issues not present in private automobiles. One issue relates to blending braking response derived from the drivetrain braking system where it supports a compression release engine brake function (commonly referred to as a Jake brake or a compression brake) in addition to regenerative braking. Addressing this issue raises other issues relating to operator control over drivetrain braking power.
An internal combustion (IC) engine functioning as a compression brake operates as an air pump which retards the (normally forward) motion of a vehicle. A compression brake is implemented on a diesel cycle IC engine by altering operation of the engine intake and exhaust valves and cutting off fuel flow to the engine. The operation can be described with reference to a single cylinder with both intake and exhaust valves closed, the piston nearing the top of its travel in the cylinder (termed “top dead center”) and a charge of compressed air in the cylinder. As the piston reaches the top of its travel the exhaust valve(s) open and compressed air is discharged to the exhaust manifold. The exhaust valve(s) then close and the piston moves toward the bottom of its travel in the cylinder pulling against an increasing vacuum in the cylinder. At the bottom of the piston's travel the intake valve(s) open and air is drawn into the cylinder. The intake valve then closes and the piston travels upwardly compressing the air for another cycle. Energy to force movement of the piston comes from the vehicle's forward motion. The IC engine remains mechanically coupled to the vehicle's drive wheels so the vehicle's momentum provides the energy to back drive the engine to force the cycle. In a typical non-hybrid vehicle equipped with an IC engine modified to operate as a compression brake, an operator can select whether the system is operational and the number of cylinders of the IC engine to be used for braking (e.g., 2, 4 or 6 cylinders) by operation of switches mounted in-side the cab. Thus compression braking is not the default braking mode for a vehicle but is engaged by the driver/operator. When engaged it typically activates upon the driver removing pressure from the vehicle accelerator pedal and does not require depression of the brake pedal. Depression of the brake pedal continues to engage the vehicle's service brakes.
Contemporary hybrid vehicles equipped with a parallel hybrid electric drivetrain use an electrical machine having two modes of operation. One mode is the traction mode where the electrical machine operates as a traction motor drawing power from a storage (traction) battery to provide traction (motive) power for the vehicle. In the other mode the electrical machine functions as an electrical generator which may be back driven from the vehicle's drive wheels to generate electricity, or which may be driven by the IC engine to generate electricity. Power generated by the electrical machine is stored in the vehicle's fraction battery where it can be drawn upon the supply power to the electrical machine in its traction mode. Using the electrical machine as a back driven generator slows the vehicle while recapturing energy which would otherwise be lost as heat. Like the engine compression brake the regenerative braking system provides braking through the vehicle's drive train. While it is known for regenerative braking to be built so that it engages upon release of the accelerator pedal (first mode), regenerative braking usually engages only upon depression of the brake pedal (second mode).
It might appear that regenerative braking could readily displace engine compression braking by provision of operator controls allowing selection of whether regenerative braking is engaged upon release of the accelerator pedal or upon depression of the brake pedal. While it is true that regenerative braking can usually partially displace use of the compression brake it frequently cannot fully replace it. This is due to limitations frequency inherent to a regenerative braking system. For example, regenerative braking generally does not supply as much braking torque as compression braking so compression braking remains a useful supplemental braking source which spares the use of the service brakes. In addition, regenerative braking may not be available if the vehicle's traction battery is fully charged and there is nowhere to store the power. Still, using regenerative braking is given priority since energy absorbed by the compression brake is wasted. Cooperative operation of the elements of the drivetrain braking system is dynamic, varying considerably with conditions.
Issues can arise relating to both modes of hybrid regeneration on a vehicle equipped for compression braking in that either mode can coincide with operation of the compression brake and with operation of the vehicle's service brakes. The potential exists for the loss of kinetic energy that could have been applied to generating electrical power if braking is provided by the compression brake or the service brakes. Driver input mitigates some of these issues.